使用spring boot通常使用spring-boot-starter-mail进行邮件的发送。当进行邮件群发的话，如果一个收件人的地址错误，会导致所有邮件都发送失败。因此我们需要在邮件发送失败的时候把错误的收件人移除，重新发送。
当邮件发送失败的时候会抛出MailSendException，异常信息中包含错误的收件人信息。
主要代码如下：
private void sendMail(List<String> mailList, MimeMessageHelper message){
try{
this.mailSender.send(message.getMimeMessage());
}catch (MailSendException e){
Set<String> tmpInvalidMails = getInvalidAddress(e);
// 非无效收件人导致的异常，暂不处理
if (tmpInvalidMails.isEmpty()){
logger.error(e.getMessage());
return;
}
mailList.removeAll(tmpInvalidMails);
if(mailList.isEmpty()){
logger.error("邮件发送失败，无收件人" + e.getMessage());
return;
}
message.setTo(mailList.toArray(new String[0]));
sendMail(mailList, message)
}
}
捕获邮件发送失败的异常，首先判断是否是收件人无效导致的异常。从异常中解析无效收件人，收件人例表中移除无效的收件人，重新发送邮件。
如何从从异常中获取无效收件人，首先看下JavaMailSenderImpl这个类的doSend方法
protected void doSend(MimeMessage[] mimeMessages, @Nullable Object[] originalMessages) throws MailException {
Map<Object, Exception> failedMessages = new LinkedHashMap<>();
Transport transport = null;
try {
for (int i = 0; i < mimeMessages.length; i++) {
...
// Send message via current transport...
MimeMessage mimeMessage = mimeMessages[i];
try {
...
}
catch (Exception ex) {
Object original = (originalMessages != null ? originalMessages[i] : mimeMessage);
failedMessages.put(original, ex);
}
}
}
finally {
try {
if (transport != null) {
transport.close();
}
}
catch (Exception ex) {
if (!failedMessages.isEmpty()) {
throw new MailSendException("Failed to close server connection after message failures", ex,
failedMessages);
}
else {
throw new MailSendException("Failed to close server connection after message sending", ex);
}
}
}
if (!failedMessages.isEmpty()) {
throw new MailSendException(failedMessages);
}
}当邮件发送过程中遇到异常会保存到failedMessages中，我们需要从中解析收件人无效导致的异常。
接着继续看源码SMTPTransport的rcptTo方法，会去校验每个收件人，通过向服务器发送RCPT TO:<地址>，根据响应码来判断收件人是否有效。
protected void rcptTo() throws MessagingException {
List<InternetAddress> valid = new ArrayList();
List<InternetAddress> validUnsent = new ArrayList();
List<InternetAddress> invalid = new ArrayList();
...
int k;
for(k = 0; k < this.addresses.length; ++k) {
sfex = null;
InternetAddress ia = (InternetAddress)this.addresses[k];
String cmd = "RCPT TO:" + this.normalizeAddress(ia.getAddress());
if (dsn) {
cmd = cmd + " NOTIFY=" + notify;
}
this.sendCommand(cmd);
int retCode = this.readServerResponse();
switch(retCode) {
case 250:
case 251:
valid.add(ia);
...
break;
case 450:
case 451:
case 452:
case 552:
...
validUnsent.add(ia);
...
break;
case 501:
case 503:
case 550:
case 551:
case 553:
...
invalid.add(ia);
...
break;
default:
if (retCode >= 400 && retCode <= 499) {
validUnsent.add(ia);
} else {
...
invalid.add(ia);
}
...
}
}
if (sendPartial && valid.size() == 0) {
sendFailed = true;
}
int lrc;
if (sendFailed) {
this.invalidAddr = new Address[invalid.size()];
invalid.toArray(this.invalidAddr);
this.validUnsentAddr = new Address[valid.size() + validUnsent.size()];
k = 0;
for(lrc = 0; lrc < valid.size(); ++lrc) {
this.validUnsentAddr[k++] = (Address)valid.get(lrc);
}
for(lrc = 0; lrc < validUnsent.size(); ++lrc) {
this.validUnsentAddr[k++] = (Address)validUnsent.get(lrc);
}
}
...
if (sendFailed) {
this.logger.fine("Sending failed because of invalid destination addresses");
...
throw new SendFailedException("Invalid Addresses", (Exception)mex, this.validSentAddr, this.validUnsentAddr, this.invalidAddr);
}
}当收件人无效发送失败会抛出SendFailedException异常，异常中包含收件人是否有效的信息。
因此我们只要从failedMessages查找是否含有SendFailedException，然后从SendFailedException直接得到无效的收件人信息。代码如下：
private Set<String> getInvalidAddress(MailSendException e){
Set<String> mails = new HashSet<>();
for(Exception exception: e.getFailedMessages().values()){
if(exception instanceof SendFailedException){
for(Address address: ((SendFailedException) exception).getInvalidAddresses()){
mails.add(address.toString());
}
}
}
return mails;
}